The present invention relates to magnetic resonance imaging systems; and more particularly to equipment for detecting errors, measuring performance and calibrating such systems.
Any nucleus which possesses a magnetic moment attempts to align itself with the direction of the magnetic field in which it is located. In doing so, however, the nucleus precesses around this direction at a characteristic angular frequency (Larmor frequency) which is dependent on the strength of the magnetic field and on the properties of the specific nuclear species (the magnetogyric constant .gamma., of the nucleus). Nuclei which exhibit this phenomena are referred to herein as "spins".
When a substance, such as human tissue, is subjected to a uniform magnetic field (polarizing field B.sub.0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. A net magnetic moment M.sub.z is produced in the direction of the polarizing field, but the randomly oriented magnetic components in the perpendicular, or transverse, plane (x-y plane) cancel one another. If, however, the substance, or tissue, is subjected to a radio frequency field (excitation field B.sub.1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, M.sub.z, may be rotated, or "tipped", into the x-y plane to produce a net transverse magnetic moment M.sub.t, which is rotating, or spinning, in the x-y plane at the Larmor frequency. The practical value of this phenomenon resides in the signal which is emitted by the excited spins after the excitation signal B.sub.1 is terminated. There are a wide variety of measurement sequences in which this nuclear magnetic resonance ("NMR") phenomena is exploited.
When utilizing NMR to produce images, a technique is employed to obtain NMR signals from specific locations in the subject. Typically, the region which is to be imaged (region of interest) is scanned by a sequence of NMR measurement cycles which vary according to the particular localization method being used. To perform such a scan, it is necessary to elicit NMR signals from specific locations in the subject. This is accomplished by employing magnetic fields (Gx, Gy, and Gz) which have the same direction as the polarizing field B.sub.0, but which have a gradient along the respective x, y and z axes. By controlling the strength of these gradients during each NMR cycle, the spatial distribution of spin excitation can be controlled and the location of the resulting NMR signals can be identified. The received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
The NMR system has to accurately generate the various magnetic fields in order to create an image of the region of interest. One technique for detecting inaccuracies in the generated fields and in the detection of NMR signals involves placing a test sample of a known material at a defined location within the NMR system. After exciting the test sample, NMR signals therefrom are detected and analyzed to determine if the received signals conform with the predicted response from the test sample. Any deviation from the predicted response provides information that is useful in determining the error in the system that caused the deviation and the compensation measures to be taken.
Previous test samples were excited by a small RF coil placed immediately around the sample. In order to minimize the test RF coil being affected by signals from other coils within the magnet assembly, a resistor was connected in series with the test coil to reduce the quality factor Q of the coil. Even so, the test coil response was very sensitive to the position within fields generated by the NMR system. While this approach worked well in previous systems, it lowered the signal to noise ratio of the test coil signal and decreased the test coil's overall sensitivity to NMR signals.
Certain NMR systems have a plurality of input channels for processing NMR signals. With prior test coils, separate test procedures had to be run for each channel as the poor signal to noise ratio and sensitivity of the test coil permitted the test coil to be connected to only one channel at a time.